1. Field of the Invention
The present invention relates to an image forming apparatus, having a plurality of image forming sections, for forming an image by the electrophotographic method or the electrostatic recording method, suitably applicable when improving productivity and machine service life regarding image forming upon forming image read out from a color mixed black-white original (original comprising a mixture of color and monochromatic originals) or from color-monochromatic data read out from a computer onto a recording medium such as paper.
2. Description of the Related Art
A conventional color image forming apparatus has been based on a process of multi-transferring images onto a recording medium while sequentially conveying a plurality of image forming sections on a conveyor belt, or multi-transferring images on an intermediate transfer belt and then transferring all at once on a recording medium.
A color image forming apparatus of this type has control means that switches over the mode between a color image forming mode for forming a color image on a recording medium in response to a read image of the original, and a monochromatic image forming mode for forming a monochromatic image on the recording medium. In such a color image forming apparatus, when a single color original and monochromatic originals are processed at the same time, image forming is performed in the color image forming mode, including for the monochromatic originals, since the single color original requires driving of image forming sections for colors (a yellow recording unit, a cyan recording unit, and a magenta recording unit), even though driving of color image forming sections are not usually required for image forming of a monochromatic original. This reduces the service life of the color image forming sections.
For the purpose of coping with the above-mentioned problem, Japanese Patent Application Laid-Open No. 10-285421 proposes a technique of switching over the mode between a color image forming mode and a monochromatic image forming mode in page units, and processing a monochromatic original in the monochromatic image forming mode, thereby improving durability.
The mode is switched over between the color image forming mode and the monochromatic image forming mode. When mode switching is frequent, therefore, switching takes much time, thus leading to a problem of decreased productivity.
In Japanese Patent Application Laid-Open No. 2001-305818 the frequency of mode switching between the color image forming mode and the monochromatic image forming mode is reduced and the productivity of image forming is improved by determining the number of monochromatic images for which continuous image forming is currently underway, and switching over the mode from the color image forming mode to the monochromatic image forming mode on the basis of the result of such a determination. However, while an increase in the number of formed images determined as described above improves productivity, this increase reduces the machine service life of the color image forming sections. On the other hand, a smaller number of formed images determined as above leads to a higher switching frequency between the color image forming mode and the monochromatic image forming mode, thus permitting achievement of a longer service life. This, however, leads to a lower productivity.
In order to achieve a running cost at or below a prescribed level, while maintaining a high productivity of image forming, it has been necessary to provide sufficient room for machine service life. This has required the use of expensive parts each having a sufficient service life for all portions that may be deteriorated when forming images in the color image forming mode. In order to achieve a prescribed running cost when using short-life parts, in contrast, it has been impossible to improve productivity of image forming.